Catalyst including transition metal has been widely used for olefin polymerization. For example, German Patent Nos. 2,608,933 and 3,007,725 have disclosed that metallocene compound consisting of Group 4 transition metal such as zirconium, titanium or hafnium, and a ligand having cyclopentadienyl structure can be used as a catalyst for olefin polymerization in the presence of an activator such as methylaluminoxane. Examples of the ligands include substituted or unsubstituted cyclopentadiene, indene, and fluorene. Various metallocene compounds have been developed to prepare catalyst systems for the olefin polymerization. It has also been known that changes of chemical structures of the metallocene compounds may have significant effects on the suitabilites of the compounds as catalyst. For example, activities, stereospecificities, and stabilities of the catalyst and physical properties of the polymer obtained by the polymerization depend on the sizes and positions of substituents bonded to cyclopentadienyl ligands. Specific examples of the meallocene compounds include bis(alkylcyclopentadienyl)zircornium dichloride (wherein, alkyl represents methyl, ethyl, isopropyl, tert-butyl or trimethylsilyl.) [J. Chem. Soc. Dalton Trans., 805(1981)], bis(pentamethylcyclopentadienyl)zirconium dichloride [J. Amer, Chm. Soc., 100, 3078(1978)], (pentamethylcyclopentadienyl) (cyclopentadienyl)zirconium dichloride [J. Amer, Chm. Soc., 106, 6355(1984)], bis(di, tri, or tetra alkyl-cyclopentadienyl)zirconium dichloride [U.S. Pat. No. 4,874,880], non-bridged metallocene compounds having substituted indenyl ligands [U.S. Pat. No. 5,780,659], metallocene compounds having mono substituted cyclopentadienyl ligand [German Patent No. 4,312,270], and so on. Recently, it have been discovered that the catalyst system including the metallocene compound, which uses zirconium as the transition metal, and methyl aluminoxane (MAO) activator has high activities in olefin polymerization (European Patent No. 129,368, U.S. Pat. Nos. 4,874,880, and 5,324,800, and Makromol. Chem. Rapid Commun., 4, 417(1983) et al.).
When the catalyst system including only one kind of the metallocene is used for the olefin polymerization, the produced polyolefin has a good mechanical strength and transparency due to the narrow molecular weight distribution (Mw/Mn) of about 2. But the produced polyolefin is not suitable for processing, and the surface of the processed product is not desirable in quality. To improve the physical properties and processability of the produced polyolefin, U.S. Pat. Nos. 4,530,914 and 4,975,403 disclosed a method of preparing polyolefin having wide molecular weight distribution by using two or more metallocenes and cocatalyst. In this method, polyolefins of different properties are produced at the same time due to the different properties of the metallocenes. However, in the method, the reaction conditions are complicated, the control of the molecular weight distribution is difficult, and the reaction conditions should be very carefully controlled to obtain polyolefin having the desirable molecular weight distribution.
Generally, the catalyst system including aluminoxane or organic boron compound and metallocene compound shows best catalytic activity at the polymerization temperature of 60 to 80° C. However, the activity of the catalyst system decreases as the reaction time passes at such a high temperature, and, therefore, the polymerization productivity decreases, the physical properties of the produced polymer are deteriorated, and the produced polymer is not suitable for processing. If the polymerization temperature decreases, the molecular weight of the produced polymer can be increased, but the activity of the catalyst decreases.
In order to solve these problems and to increase the molecular weight distribution of the produced polyolefin, various metallocene catalysts including two or more metals have been developed. For example, U.S. Pat. No. 5,753,577 disclosed a multinuclear metallocene compound including two Group 4 transition metals having oxidation state of 3, in which the two transition metals are directly bonded, and the ligands of the two transition metals are also directly bonded. However, the polymer produced with this multinuclear metallocene catalyst is not satisfactory due to the limitation of its molecular weight. U.S. Pat. No. 5,442,020 disclosed a multinuclear metallocene compound which is produced by reacting Group 4 transition metal compounds and cyclopentadienyls which are connected with alkylene or silylene, and the multinuclear metallocene compound is used for ethylene polymerization, propylene polymerization, and ethylene/α-olefin copolymerization. However, the alkylene or silylene group should be used for preparing the catalyst, and therefore, the method of preparing the catalyst is complicated. U.S. Pat. No. 5,627,117 disclosed a multinuclear metallocene compound which is produced by reacting Group 4 to Group 8 transition metal compounds and cyclopentadienyls which are connected with alkylene, silylene, or divalent Ge or Sn, and the multinuclear metallocene compound is used for ethylene polymerization, propylene polymerization, and ethylene/α-olefin copolymerization. However, the metallocene compound is not satisfactory due to the low activity at high temperature. U.S. Pat. No. 6,010,974 disclosed a multinuclear metallocene compound which is produced by reacting Group 4 transition metal compounds and cyclopentadienyls which are connected with alkylene or silylene, and the multinuclear metallocene compound is used for styrene polymerization. However, the metallocene compound is not satisfactory since the compound is suitable only for polymerization of styrene. Besides, various multinuclear metallocene catalysts are disclosed in U.S. Pat. Nos. 5,585,508, 5,830,958, 5,892,079, 5,962,359, 5,986,024, 6,262,197, 6,344,528, and 6,642,400, and Korean Laid-Open Patent 2002-0063506 (Application No. 10-2002-0004142).